1. Field of the Invention
The invention relates to steerable rotary drilling systems and provides, in particular, methods and apparatus for the transmission of data from the bottom hole assembly of such a drilling system, either to the surface or to another downhole location.
2. Setting on the Invention
When drilling or coring holes in subsurface formations, it is sometimes desirable to be able to vary and control the direction of drilling, for example to direct the borehole towards a desired target, or to control the direction horizontally within the payzone once the target has been reached. It may also be desirable to correct for deviations from the desired direction when drilling a straight hole, or to control the direction of the hole to avoid obstacles.
Rotary drilling is defined as a system in which a bottom hole assembly, including the drill bit, is connected to a drill string which is rotatably driven from the drilling platform at the surface. Hitherto, fully controllable directional drilling has normally required the drill bit to be rotated by a downhole motor. The drill bit may then, for example, be coupled to the motor by a double tilt unit whereby the central axis of the drill bit is inclined to the axis of the motor. During normal drilling the effect of this inclination is nullified by continual rotation of the drill string, and hence the motor casing, as the bit is rotated by the motor. When variation of the direction of drilling is required, the rotation of the drill bit is stopped with the bit tilted in the required direction. Continued rotation of the drill bit by the motor then causes the bit to drill in that direction.
Although such arrangements can, under favorable conditions, allow accurately controlled directional drilling to be achieved using a downhole motor to drive the drill bit, there are reasons why rotary drilling is to be preferred, particularly in long reach drilling.
Accordingly, some attention has been given to arrangements for achieving a fully steerable rotary drilling system.
The present invention relates to a steerable rotary drilling system of the kind where the bottom hole assembly includes, in addition to the drill bit, a modulated bias unit and a control unit, the bias unit comprising a number of hydraulic actuators at the periphery of the unit, each having a movable thrust member which is hydraulically displaceable outwardly for engagement with the formation of the borehole being drilled, each actuator having an inlet passage for connection, through a control valve, to a source of drilling fluid under pressure, the operation of the valve being controlled by the control unit so as to modulate the fluid pressure supplied to the actuators as the bias unit rotates.
Although there are preferably provided a plurality of actuators spaced apart around the periphery of the bias unit, the invention also relates to systems where the bias unit has only a single actuator.
In one mode of operation, when steering is taking place, the control unit causes the control valve to operate in synchronism with rotation of the bias unit, and in selected phase relation thereto whereby, as the bit rotates, the or each movable thrust member is displaced outwardly at the same selected rotational position so as to bias laterally the bias unit and the drill bit connected to it, and thereby control the direction of drilling.
A steerable rotary drilling system of this kind is described and claimed, for example, in British Patent Specification No. 2259316. One form of control unit for use in such a system is described and claimed in British Patent Specification No. 2257182.
In the course of operating a steerable rotary drilling system it may be necessary to transmit to the surface data giving information on the operating parameters of the bottom hole assembly. For example, it may be required to transmit information concerning the status of the equipment including the control unit and bias unit, or information concerning the command status, that is to say the instructions which the control unit is giving to the bias unit. Furthermore, it may be required to transmit to the surface survey information regarding the azimuth and inclination of part of the bottom hole assembly, or the roll angle of the control unit, or geological information.
Such information may in some cases be transmitted to another downhole location, either for onward transmission to the surface by other means, or to control operation of another piece of downhole equipment.
There are various well known methods currently employed for transmitting data from a bottom hole assembly to the surface, since such requirement also exists for directional drilling using a downhole motor as well as for measurement-while-drilling (MWD) systems generally. One method commonly used is to transmit data to the surface as a sequence of pulses transmitted upwardly through the drilling fluid by a specially designed pulser which is included in the bottom hole assembly and responds to data signals from appropriate sensors in the assembly. In a common form of pulser, known as a negative pulser, a negative pulse (i.e. a pulse causing a drop in fluid pressure) is generated by the temporary diversion to the annulus of a proportion of the drilling fluid passing downwardly through the drill string to the drill bit. However, there are difficulties in using such a pulser in a steerable rotary drilling system of the kind first referred to. For example, a negative pulser requires the provision of mechanical hardware mounted on the drill collar to effect the diversion of fluid through a passage in the drill collar leading to the annulus. Such hardware also requires a power source for its operation, which must also be mounted on the drill collar.
In the preferred embodiment of the system to which the present invention relates, however, the control unit is a roll stabilized instrument carrier which is rotatable relative to the drill collar. This makes it difficult to pass power and control instructions from the control unit to a relatively rotating pulser hardware on the drill collar. It is possible to mount on the control unit a positive pulser of the kind where pulses are generated by choking or cutting off part of the flow of drilling fluid along the drill string but, again, there are practical difficulties in this.
The present invention is based on the real sation that the bias unit itself has certain of the characteristics of a negative pulser, in that during its operation it diverts to the annulus a varying proportion of the drilling fluid which would otherwise pass to the drill bit. The invention therefore lies, in its broadest aspect, in using the bias unit itself as a pulser for transmitting data pulses to the surface or to another downhole location.
The term "pressure pulse" will be used to refer to any detectable change in pressure caused in the drilling fluid, regardless of the duration of the change, and is not necessarily limited to temporary changes in pressure of short duration.